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Oxidation of the unambiguously characterized iridium(III)-enediamido complex IrCp∗(RNCHCHNR), R = 2,6-dimethylphenyl, with six cyclically arranged π electrons was investigated using EPR and UV–vis spectroelectrochemistry. In contrast to a corresponding iminocatecholato system reported recently by Rauchfuss et al. [27] the two one-electron oxidation steps are not completely reversible, depending on the solvent. We attribute the electrochemical behavior observed in weakly coordinating dichloromethane and propylene carbonate solvents to an enhanced propensity for ligand addition: while neutral IrCp∗(RNCHCHNR) remains coordinatively unsaturated with the strongly σ and π donating enediamido(2−) ligand mitigating the electron deficit, the oxidation to the less donating radical anion ligand (RNCHCHNR) - or even to neutral 1,4-diazabutadiene is considered to induce a strong tendency for quasi-hexacoordination in [IrCp∗(L)(RNCHCHNR)], L = solvent, halide, substrate. The presence of excess chloride thus leads to formation of the precursor complex ion [IrCp∗ClRNCHCHNR)]+ after oxidation. EPR spectroscopy of the one-electron oxidation intermediate [IrCp∗(RNCHCHNR)]+ was successful only for a frozen solution in propylene carbonate/0.1 M Bu4NPF6 which revealed an axial signal with sizeable g anisotropy. DFT calculation results for [IrCp∗(RNCHCHNR)]n, n = 0, +, 2+ confirm the available structural and spectroelectrochemical data (UV–vis, EPR).

3,6-Bis(2-thienyl)-1,2,4,5-tetrazine (bttz) reacts with trans-Pt(dmso)2(mes)2, mes = mesityl = 2,4,6-trimethylphenyl, under twofold cyclometallation to yield structurally characterized (μ-bttz-2H+)[Pt(dmso)(mes)]2 with uncoordinated thiophene sulfur atoms and bttz deprotonated in the 3,3′ positions. The structural features include cis-positioned carbanionic ligands, twisted mesityl substituents, S-coordinated dmso ligands with the S O bonds lying in the molecular plane, shortened inter-ring bonds, and rather short Pt–C bonds at 1.998(9)/2.00(1) A (Pt–Cmes) and 1.985(9)/1.99(1) A (Pt–Cbttz-2H+). Reversible reduction to {(μ-bttz-2H+)[Pt(dmso)(mes)]2} − causes a high-energy shift of the charge transfer bands and the appearance of an unresolved EPR signal at g = 1.9905.

For the first time a complete set of tricarbonylhalidorhenium(I) complexes (Hal = F, Cl, Br, I) has been studied in a systematical fashion by example of (abpy)Re(CO)3(Hal), abpy = 2,2′-azobispyridine. Crystal structures of chloride, bromide and iodide analogues are now available, showing increasing planarization of the abpy ligand in that order. Cyclic voltammetry, EPR, IR and UV/Vis spectroelectrochemistry of the reduced forms [(abpy)Re(CO)3(Hal)] − illustrate that the four halide complexes differ only partially in their properties. The strongest deviations are observed for [(abpy)Re(CO)3F] − which is distinguished by the widest electrochemical potential range but most pronounced chemical lability. In the EPR spectrum the fluoride exhibits the highest isotropic g value (2.0085) and the lowest rhenium coupling constant, which is of the same magnitude (2 mT) as the detectable 19F hyperfine splitting.

The TCNX ligands TCNE (tetracyanoethene) and TCNQ (7,7,8,8-tetracyano-p-quinodimethane) react instantaneously with (C5R5)2(CO)2Ti, R=H or Me, to yield highly air-sensitive mononuclear complexes (C5R5)2(CO)Ti(TCNX) of which the soluble species (R=Me) were characterized also in the oxidized and reduced forms through cyclic voltammetry, EPR, IR and UV–vis spectroelectrochemistry. While oxidation at rather low potentials yields labile carbonyltitanium(IV) species of the TCNX− ligands, the reduction occurs stepwise at unusually negative potentials, first on the ligand (to yield coordinated TCNX2−) and then on the metal (to form TiII). For the neutral complexes (C5R5)2(CO)Ti2+q(TCNX−q) the results support a rather large amount of charge transfer 1

The organometallic complex CpFe(CO)2CH3 is included in a β-cyclodextrin cavity, retains its redox activity and undergoes CO insertion into the methyl–metal bond. The reaction inside the cavity suppresses undesirable loss of released CO from the reaction site. This simplifies the mechanism of the bond-insertion reaction in the presence of cyclodextrin. Further enhancement is observed in presence of free CO, which can penetrate to the reaction site inside the cyclodextrin cavity.

The syntheses of the water-soluble clusters [Os3(CO)9(μ-η2-(LH))(μ-H)L′] (L=3-amino quinoline, L′=Na3[P(C6H4SO3)3] 1; L=3-amino quinoline, L′=[P(OCH2CH2NMe3)3]I3, 2; L=3-(2-phenyl acetimido) quinoline, L′=[P(OCH2CH2NMe3)3]I3, 3; L=phenanthridine, L′=[P(OCH2CH2NMe3)3]I3, 4) are reported. The products have been fully characterized by 1H-, 13C VT-NMR and LC–TOF-MS. The effect of pH and concentration on intermolecular aggregation in water has been investigated. The interactions of these clusters with DNA have been studied using the plasmid super coiled DNA relaxation test in a 1% electrophoresis agarose gel. Band retardation due to cluster binding was observed for the positively charged clusters 2–4, but not for the negatively charged 1. The relative binding affinities were in the order 2

The compounds [(Cym)OsCl(dxpf)](PF6), Cym=p-cymene and dxpf: 1,1′-bis(diphenylphosphino)ferrocene (dppf), 1,1′-bis(diethylphosphino)ferrocene (depf) or 1,1′-bis(diisopropylphosphino)ferrocene (dippf), were synthesized and characterized by NMR (1H, 31P) and, in the case of [(Cym)OsCl(dppf)](PF6), by X-ray structure analysis of the acetonitrile solvate. EPR and UV–vis spectroelectrochemistry indicate the formation of an osmium(II)–ferrocenium species on reversible one-electron oxidation. The second oxidation and the reduction are electrochemically irreversible.

The complexes (OC)3ClRe(abpy) (1) and [(OC)3ClRe]2(abpy) (2) with abpy=2,2′-azobispyridine were structurally characterized and studied by spectroelectrochemistry in the UV–vis and in the IR carbonyl stretching region. Compound 1 exhibits a s-cis/E/s-trans configurated abpy ligand with metal coordination to one pyridyl and one azo function and one non-bonding ReN interaction at 3.293 A to the second pyridyl nitrogen atom. The dinuclear complex 2 with two azoimine-coordinated metal centers is distinguished by the trans arrangement of the chloride ligands with respect to the Re2(μ-abpy) plane, the ReRe distance is 5.033(7) A. The azo bond lengths at 1.272(9) (1) and 1.304(10) A (2) indicate substantial π back donation from the rhenium(I) centers into the π*(abpy) orbital. Spectroelectrochemistry reveals the successive occupation of the π*(abpy) orbital by electrons; metal-to-ligand charge transfer features are shifted to higher energies and carbonyl stretching bands to lower wavenumbers. The EPR spectra of both radical anion intermediates 1 − and 2 − are dominated by very similar 185,187Re hyperfine splitting, ruling out a mixed-valent formulation. The second electron uptake is followed by the rapid loss of one chloride, leading to an unsymmetrical species in the dinuclear system.

The compound fac -Re(bpy)(CO) 3 (COCH 3 ) ( 1 ) (bpy=2,2′-bipyridine) with three different kinds of π-acceptor ligands coordinated to rhenium(I) has been investigated by infrared spectroelectrochemistry to reveal the occupation of the π*(bpy) MO on one-electron reduction to 1 − with negligible participation of the π-accepting acetyl group. The results are discussed in relation to the spectroscopy (NMR, IR) and the crystal structure analysis of the neutral complex 1 , which reveals a short CO(acetyl) (1.157(12) A) and a long ReC(acetyl) bond (2.245(12) A) and an orientation of the ReC(O)CH 3 plane parallel to the C 2 axis of the coordinated bpy. The result is an orthogonal arrangement of d xz /π*(bpy) and d xy /π*(acetyl) orbitals. This situation stands in distinct contrast to the structure reported for the complex salt [Re(L)(CO) 3 (COCH 3 )](PPN) (PPN + =bis(triphenylphosphoranylidene)ammonium) with a cyclometallated 2-phenylpyridine ligand L. The crystal structures of the related complexes [M(bpy)(CO) 4 ](OTf), M=Re and Mn, were determined for comparison.

Two organometallic complex fragments with 5d6 configurated metal centres of different character, electrophilic [IrIIICp*Cl]+ and π-donating [ReI(CO)3Cl], were used as probes in compounds containing 1,3-dimethyllumazine (DML), 1,3-dimethylalloxazine (DMA), 2-pivaloylpterin (PP) and 6-methyl-2-pivaloylpterin (MPP) as biochemically relevant ligands. Evidence from spectroscopy (NMR, IR, UV–vis) in aprotic solvents points in most cases to the O4 and N5 atoms as chelate donors for metal binding. For [(DMA)IrIIICp*Cl](PF6) this was substantiated by a crystal structure analysis. With PP, however, an iridium(III) compound with pivaloyl-coordinated metal was obtained. Whereas both the IrIII and ReI complexes of the good π acceptor ligand DMA can be reduced reversibly and thus studied by (spectro)electrochemistry (IR, UV–vis, EPR: ligand-centred one-electron reduction), the DML system exhibits such behaviour only for the IrIII species. The complexes of the 2-pivaloylpterins showed only irreversible reduction.

The dinuclear complexes [Cp*ClM( μ -L)MClCp*](PF 6 ) 2 , M=Rh or Ir, L=3,6-bis(2-pyridyl)-1,2,4,5-tetrazine (bptz) or 2,5-bis(phenyliminoethyl)pyrazine (bpip), are reduced in several chemically reversible steps by up to six electrons to the species [Cp*M( μ -L)MCp*] n − . UV–vis/NIR spectroelectrochemistry and EPR of the paramagnetic states were used to identify the various intermediates. The complexes clearly show a reversible, ligand-centered one-electron reduction (E) preceding the first chloride-dissociative metal reduction step (EC). Metal–metal interaction via the bridging π acceptor ligand L causes a splitting of 310–710 mV between the potentials for the two Cl − -dissociative steps. The second chloride release occurs in EC+E fashion for L=bpip but in a two-electron process for L=bptz. The M II M I mixed-valent species [Cp*M( μ -L)MCp*] + could be identified via long-wavelength bands from intervalence charge transfer (IVCT) transitions. All complexes containing at least one chloride-free Cp*M group display intense long-wavelength absorption bands. The iridium complexes are distinguished by more negative potentials of the [Cp*Ir]-containing forms, by slower formation of the M 2 I,II mixed-valent intermediate, by larger g anisotropy of the paramagnetic forms, and by triplet absorption features in the UV–vis electronic spectra.

In contrast to the rapid and chemically reversible two-electron ECE′ reductive elimination reaction[(C 5 Me 5 )ClM(bpy)] + + 2e − → (C 5 Me 5 )M(bpy)+Cl − ,M=Rh or Ir, the analogous cobalt system exhibits two separate one-electron steps (EC + E′ process) with a persistent, EPR-spectroscopically characterized cobalt(II) intermediate [(C 5 Me 5 )Co(bpy)] + . Within the series of coordinatively unsaturated homologous species (C 5 Me 5 )M(bpy), the cobalt derivative exhibits the smallest and the iridium homologue the largest metal(I)-to-bpy electron transfer in the ground state, as evident from electrochemical potentials and long-wavelength absorption data. A comparison within that homologous series indicates why the rhodium system, with its intermediate position, is most suitable for hydride transfer catalysis.

The title complexes, isolated as trans,mer -configurated salts [(PR 3 ) 2 (CO) 3 (R′-pz)M](PF 6 ), display a strong π interaction between the zerovalent metal center and the excellent π acceptor ligands R-pz + . The solvatochromism of the long-wavelength metal-to-ligand charge transfer (MLCT) absorption as analyzed in particular for [(PCy 3 ) 2 (CO) 3 (CH 3 (CH 2 ) 17 -pz)W](PF 6 ) correlates not with established MLCT parameters but with the donor numbers of the solvents. The seemingly reversible one-electron oxidation and reduction processes were studied by spectroelectrochemical techniques with regard to EPR, carbonyl vibrational and UV-vis/NIR spectroscopic features. Whereas oxidation leads to labile M(I) species with 17 valence electrons, the reduction is centered at the R′-pz + ligand and causes a rapid reversible dissociation of one PR 3 group from the metal (EC rev process). Both EPR and IR data suggest pentacoordination at the 16 + δ valence electron centers in the neutral species [(PR 3 )(CO) 3 (R′-pz)M].